The present invention relates to a composition for producing a co-fired glass-ceramic substrate having patterns of a low electrical-resistance conductor such as Ag, Ag-Pd, Au and/or Cu, for example.
The said substrate mounts a variety of chip components including semiconductor devices, coils, capacitors, resistors, and others, and said chip components mutually are connected by the conductor.
Substrates containing both mullite crystalline phase and cordierite crystalline phase are preferably used for electronic packages incorporating parts composed of aluminum nitride (AlN) for mounting semiconductor devices.
Conventionally, alumina (Al.sub.2 O.sub.3) is used for composing substrates built in computers and consumer-use electrical appliances. To sinter alumina, extremely high temperature approximating to 1600.degree. C. is required, and therefore, specific metallic material such as tungsten (W) or molybdenum (Mo) having high melting point is used for the patterning conductor which is subject to a firing process in conjunction with alumina. However, either of the above-cited metallic materials has a high electrical-resistance value, and therefore, Ag, Ag-Pd, Au, and Cu having a low resistance have long been desired as conductors. In response, a wide variety of arts have been proposed for providing substrates sinterable under low temperature at a temperature ranging from about 850.degree. C. to about 1050.degree. C. and capable of accommodating the above-cited low-resistance conductors thereon.
The Japanese Patent Laid-Open No. SHO61-31348 (1986) discloses an art "Ceramics sintered under low temperature", which proposes such a substrate complete with a process to mix 55%-60% of CaO-B.sub.2 O.sub.3 -SiO.sub.2 glass with 40%-45% of filler material such as alumina followed by a process to sinter the mixture at a low temperature ranging from 750.degree. C. to 850.degree. C. The Japanese Patent Laid-Open No. SHO59-83957 (1984) discloses an art "Crystallizable glass material", which proposes such a substrate complete with a process to yield frit by way of pulverizing crystallizable glass composition comprising 40%-52% by weight of SiO.sub.2, 27%-37% by weight of Al.sub.2 O.sub.3, 10%-20% by weight of MgO, 2%-8% by weight of B.sub.2 O.sub.3, 2%-8% by weight of CaO, and 0.1%-3% by weight of ZrO.sub.2, followed by a process to form frit, and a final process to sinter the formed frit under a low temperature ranging from 900.degree. C. to 950.degree. C. However, because of weak strength of glass matrix, the substrate proposed by the former art has weak strength merely being rated to be 15 through 19 kgf/mm.sup.2. Likewise, because of weak strength of cordierite sharing main crystalline phase, the substrate proposed by the latter art also has weak strength merely being rated to be 17-21 kgf/mm.sup.2. As is apparent from these prior art, because of insufficient strength, each of those preceding arts still has technical problem by way of easily incurring crack to multilayer substrates produced from poor-strength glass ceramics, thus requiring care whenever handling them.
In addition, because of low thermal conductivity, each of those conventional substrates fired under low temperature could not mount a high-speed LSI thereon in that it consumes a large amount of electric power. To solve this problem, as was disclosed in the Japanese Utility Model Laid-Open No. SHO61-149336 (1986) for example, a package incorporating composite structure was proposed. The proposed package comprises a part for mounting high-speed LSI devices thereon and the said part uses specific material having high thermal conductivity such as aluminum nitride (ALN) for example. The said package further comprises a patterned multilayer substrate fired under low temperature. It is essential for the package containing composite structure to exhibit approximate values of thermal expansion coefficient between the part having high thermal conductivity and the part of substrate fired under low temperature. It is also essential for the substrate itself to have high strength. However, according to the package based on the composite structure cited above, actual values of thermal expansion coefficient between the part having high thermal conductivity and the substrate fired under low temperature are different from each other. This in turn easily generates residual stress. And also the part of substrate has quite insufficient strength.
In terms of rough classification, conventionally known substrates subject to a firing process under low temperature are made from each of glass composition with filler, ceramic composition, and crystallizable glass composition. Of these, the glass composition with filler easily achieves the objective thermal expansion coefficient. However, the glass composition with filler has quite weak strength as mentioned above. Conversely, although the ceramic composition and the crystallizable glass composition respectively have substantial strength, neither of these can easily achieve the objective thermal expansion coefficient.
On the other hand, under the title of "Method of manufacturing porcelain based on mullite composition", the Japanese Patent Laid-Open No. HEI4-254468 (1992) discloses a method of manufacturing a substrate fired under low temperature by executing a step to prepare mixture powder comprising 15-60% by weight of mullite powder and 40-85% by weight of glass powder which enable to precipitate mullite crystalline phase, second step to be formed and a final step to fire the mixture at a temperature ranging from 850.degree. C. to 1050.degree. C., whereby the substrate results in eventually crystallizing a minimum of 40% by volume of mullite. However, the above-cited publication fails to disclose actual value of specific surface area of mullite powder and generation of crystalline phases other than the mullite phase present in the above-cited substrate. It is therefore hardly admissible that total degree of the crystallization of the substrate produced via a firing process is quite high. In consequence, even though the substrate has substantial strength at room temperature, its strength is degraded when being exposed to a temperature higher than 500.degree. C. Therefore it generates critical problem during a process to bond lead wires with the substrate.
On the other hand, the U.S. Pat. No. 4,528,275 discloses a mullite-cordierite composite ceramic which would be useful for supporting substrate for a silicon chip built in a ceramic package installed in a computer unit. The composite ceramic disclosed in the above U.S. Patent is based on a ceramic sintering technique comprising; a step to form a uniform particulate mixture of Al.sub.2 O.sub.3, MgO, SiO.sub.2, and nucleating agent, a step to sinter the prepared mixture at a temperature ranging from about 1290.degree. C. to about 1550.degree. C. to produce a sintered body incorporating mullite phase and glassy cordierite phase, a step to perform a nucleation-annealing process at a temperature ranging from about 600.degree. C. to about 800.degree. C. to nucleate the glassy cordierite phase, and a final step to perform a crystallization-annealing process to precipitate crystalline cordierite at a temperature above 1200.degree. C. and a certain temperature lower than that would generate liquid phase in the sintered body before eventually precipitating cordierite crystals. It is therefore quite apparent from the method prescribed in the U.S. Pat. No. 4,528,275 that co-firing of the above blended components in conjunction with Ag, Ag-Pd, Au, and Cu having low melting point is impracticable. As was disclosed in Example 11 of the above-cited U.S. Pat. No. 4,528,275, in order to co-fire all the blended components together, tungsten (W) having high melting point and high electrical-resistance value is compulsorily used for patterned conductor.
Furthermore, transparent glass-ceramic articles each containing mullite are disclosed in the U.S. Pat. Nos. 4,396,720, 4,519,828, and 4,526,873, respectively. The proposed glass-ceramic article is mainly used for laser material by doping Cr.sup.3+ ion therein. In addition, the proposed glass-ceramic article has been developed so that it can internally be used for a luminescent solar collector by way of being combined with a silicon photovoltaic cell. It is expressly described in the above-identified patent publications that absence of crystalline phases other than mullite phase is preferable. Above all, no technical thought has ever been disclosed in each of those U.S. Patents publications cited above in order to use the proposed transparent glass-ceramic article to compose a substrate subject to a co-firing process under low temperature.